A new post-hoc method to improve the eddy-covariance-based evapotranspiration measurements

Terrestrial evapotranspiration (ET) is the nexus of the water, energy and carbon cycles, and therefore accurate quantification of ET is important for understanding the climate and the Earth system. However, current ET estimates derived from process-based models and remotely sensed observations are subject to significant uncertainty, a major reason for which is the limited availability and quality of ground validation data. The eddy covariance (EC) technique provides an excellent opportunity for continuous ET measurements at ecosystem scales with high temporal resolution (half-hourly or hourly resolution), and nowadays eddy towers are deployed in almost all types of terrestrial ecosystems and climatic conditions. Most EC-based ET estimates, however, suffer from an energy imbalance: the sum of sensible and latent heat fluxes is often lower than the available energy (i.e. the difference between net radiation and soil heat flux). The general consensus on the causes includes instrumental bias, missing stored fluxes, different footprints for different variables, and imperfect assumptions in the eddy covariance approach.

In this presentation, we propose a generalised correction method (Zhang et al., 2023, 2024) across the site network. The method, statistical in nature, can improve the energy imbalance from ~80% to ~98% across the site network. The results are markedly better than those by the standard correction method implemented in the dataset processed by the ONEFlux pipeline, which tends to over-correct turbulent flux measurements. We further evaluate the corrected ET by comparing it with independent regional measurements in terms of spatial patterns and temporal variations, after upscaling the ecosystem-level data to the global scale using the latest FLUXCOM framework. The results show that the corrected ET-based upscaled estimates are closer to the ET derived from the water balance perspective and from the balloon-sounding observations. Our method provides a state-of-the-art alternative to improve the energy balance closure by correcting the site-level ET, and the improved global ET estimates can be of great value for water cycle studies and for model development.

References:

Zhang, W., Jung, M., Migliavacca, M., Poyatos, R., Miralles, D. G., El-Madany, T. S., . . . Nelson, J. A. (2023). The effect of relative humidity on eddy covariance latent heat flux measurements and its implication for partitioning into transpiration and evaporation. Agric. For. Meteorol., 330, 109305. doi:10.1016/j.agrformet.2022.109305

Zhang, W., Nelson, J. A., Miralles, D. G., Mauder, M., Migliavacca, M., Poyatos, R., Reichstein, M., Jung, M. (2024). A new post-hoc method to reduce the energy imbalance in eddy covariance measurements. Geophys. Res. Lett., (accecpted)